Hey, Is it possible to add a field (column) to an existing sublist from client script? The problem is that when on Invoice I select a customer it auto populates a native sublist field and removes a custom field that I added in UE script on beforeLoad

if its removing a custom field, it sounds like you have some conflicting script emptying the field you are wanting to keep. I would go to the scripts on the record and see if you can track that down.